Would you tell your boss that she’s stealing?

“I was having a few drinks with a close coworker and she told me she had a secret. She’s been stealing odds and ends from work for the last few years and selling them on eBay. She says she gets anywhere from $20 to $100 per item and nobody has noticed yet, though some people have noticed things have gone missing.
Do I keep the secret or find a way of telling our boss?”